The Enterprise is on a mission to convince the Halkan Council to allow the Federation to mine dilithium crystals on their planet. Kirk, Scotty, McCoy, and Uhura beam back up to the planet, but while doing so, an ion storm causes them to rematerialize in a parallel universe. There officers advance through assassination and the Empire plunders and pillages every planet they can find and destroys anyone that gets in its way.

Recap

Kirk, Scotty, McCoy, and Uhura with the high council of the planet Halka to negotiate dilithium mining rights. The Halkans are pacifists and believes that their crystals would be used in war, but Kirk gives them the option to reconsider while respecting their wishes. An ion storm hits the planet and Kirk calls for transport. They start to materialize on the Enterprise but there’s an energy surge. They fade out and then rematerialize... on a different Enterprise, wearing different uniforms...

Mirror Spock: I do not wish command. But if it should befall me, I suggest you remember that my operatives would avenge my death. And some of them... are Vulcans.

Mirror Sulu: (to Kirk) Mr. Spock has orders to kill you. He will succeed. Apparently, you will also have killed him, after a fierce fight. Regrettable, but it will leave me in command...

Episode Goofs

The sash Mirror Chekov is wearing appears/disappears/reappears as he gets in the turbolift.

You can DeForest Kelley’s wedding ring during the scene where he does his “I’m a doctor, not an engineer...” line.

James Doohan can be seen in the long shots on the bridge in the final scene, but disappears in the close-ups of Shatner and Kelley.

Episode References

This is the only episode in which Scotty refers to Captain Kirk as "Jim."

Analysis

What Changed in the Remastered Version
The Halkan planet and the Enterprise were replaced by nicely done CGI effects. The transition to the Mirror Universe, formerly signalled by cutting together scenes of the Enterprise transiting the planet and reversals of the same scenes, was replaced by a better special effect. The I.S.S. Enterprise itself is depicted as a slightly different looking ship - more closely resembling the original model used for the two pilots, looking slightly 'heavier', and with little sensors sticking out of the front of the engine nacelles. Mr. Kyle's agonizer displayed tiny electric shocks (suggestive of a taser) when applied and left a glow that persisted for several seconds after its removal. Chekov's punishment in the Agony Booth was blurred (and the sounds were changed slightly) to suggest the presence of an energy field within the booth.